This invention relates to television receivers capable of presenting the picture image in two different aspect ratios and, more particularly, to dual mode television receivers having the capability of coping with non-uniform phosphor aging caused by the showing of different aspect ratio pictures on a common display.
High definition television (HDTV) systems are being promulgated which by virtue of higher horizontal and vertical resolutions than found in conventional systems (e.g., NTSC) yield sharper pictures and better color. Instead of the 525 or 625 scanning lines used in conventional television, HDTV employs upward of a thousand lines and the picture is presented with a wider aspect ratio. More particularly, instead of the 4:3 aspect ratio that has been used since present television standards were established 40 years ago, most proposed HDTV formats use a 5:3 aspect ratio for the video screen. One such system proposed by CBS Inc. involves the transmission of two signals: one to provide a conventional television image on extant television sets, and the other signal, transmitted separately but simultaneously, would fill in the missing detail and add the extra scanning lines to the screen to provide a high-resolution 5:3 aspect ratio picture with 1,050 scanning lines and truer color. Viewers wanting this improved picture would require a new dual mode receiver to capture both signals and combine them into the high-quality display.
Thus, with the advent of HDTV it will be necessary to manufacture television receivers capable of reproducing NTSC as well as HDTV pictures, having different aspect ratios, on a common display, such as a cathode ray picture tube. Thus, as illustrated in FIG. 1 in which the two pictures are shown superimposed, the NTSC pictures have an aspect ratio of four units wide to three units high (4:3), while the HDTV pictures have an aspect ratio of five units wide to three units high (5:3) or greater. The 4:3 image occupies the area Y, and the 5:3 image occupies the total area including the central area Y and two equal width areas X at either side. The display on a common screen of pictures having different aspect ratios will pose the problem of uneven aging of the phosphor screen; whenever NTSC pictures are reproduced the areas X will not be illuminated whereas the phosphor in area Y will be aging due to illumination of this area. This will result in a "burn" pattern on the screen with the consequence that when an HDTV picture is displayed, the central area Y will be darker than the areas X.
The uneven phosphor aging problem presented in the dual mode receiver is very different from the screen burn-in of a picture having, for example, bright lines or other picture components remaining stationary for a long period of time, addressed by the protection circuit described in U.S. Pat. No. 4,338,623 wherein a video circuit is examined to detect how long the information has not been changed and, if a predetermined time limit is exceeded, limiting the picture brightness to a sufficiently low average value or, if so desired, to zero brightness. The problem also differs from the burning phenomenon that takes place on the photoelectric face in a video camera utilizing an imaging tube, when an object of a high luminance is exposed to the face, addressed by the burn-in prevention apparatus disclosed in U.S. Pat. No. 4,492,982. The system described in this patent for prevent burning of the imaging tube detects the function state of the imaging tube at the start of power supply thereto and controls an exposure control circuit for controlling the amount of light entering the imaging tube as a function of such detection.